1. Field of the Invention
The present invention relates to a method for bonding a lead to an electronic device, more particularly, to a method for bonding a terminal lead provided on a film carrier with electrodes of a large scale integrated circuit (LSI) chip for packing in a tape automated bonding (TAB) package.
2. Description of the Prior Art
With recent advances in the high integration methods to mount a plurality of electric/electronic components on a single semiconductor chip such as a LSI and a very large scale integrated circuit (VLSI), the cost reduction of, the high performance of, and downsizing of the electronic devices are achieved. Among such high integration methods, a bump bonding method in which an electrode of a semiconductor chip is bonded with a lead of the film carrier via a bump formed on the electrode is widely used. The film carrier is made of an elastic material such as a polyamide and is provided with a wiring pattern formed thereon. From the wiring pattern, the leads are extended for bonding with the electrodes formed on the surface of semiconductor chip. In this method, the bumps made of conductive materials such as gold and solders should be formed on electrodes before and independently from the bonding operation.
In FIGS. 11A to 11E, the steps for forming the bumps on the semiconductor wafer according to a bump-on-wafer technology, for an example of such bump bonding methods, are shown.
As shown in FIG. 11A, at the first step, a plurality of electrode elements 60 are formed on a surface of a semiconductor wafer 30 by using an electroplating technology. A conductive layer 31, so called "a barrier metal", is vapor deposited on the surface of semiconductor wafer 30 over the electrode elements 60 to cover it entirely. The conductive layer 31 is made of conductive metals such as titanium and palladium.
As shown in FIG. 11B, at the second step, an insulation layer 33 is further formed over the conductive layer 31. The insulation layer 33 is made of a photo sensitive insulation material such as a photo resist.
As shown in FIG. 11C, at the third step, a number of openings 34 are formed in the insulation layer 33 to expose the conductive layer 31 covering the electrode elements 60 by using a photo mask (not shown) having a pattern corresponding to those of the electrode elements 60.
As shown in FIG. 11D, at the fourth step, the semiconductor wafer 30, thus prepared to have openings 34, is immersed in an electrolytic solution and then the conductive layer 31 is connected to an electric source. The conductive layer 31 in the openings 34, exposed to the electrolytic solution, serves as an electrode. Thus, gold included in the electrolytic solution is deposited on the conductive layer 31 in the openings 34. Then, this deposition grows up to form a bump 54 in each opening 34.
As shown in FIG. 11E, at the fifth step, the semiconductor wafer 30 is etched to remove insulation layer 33 and the conductive layer 31 on the portions excluding the bumps 54 from the semiconductor wafer 30. Thus, the bumps 54 are completely formed on each electrode element 60, independent of each other.
In FIGS. 12A to 12C, the steps for bonding bumps 54 of a semiconductor chip 59 with leads 55 of a film carrier 56 are shown.
As shown in FIG. 12A, at the first step, the semiconductor chip 59, cut from the semiconductor wafer 30 with bumps 54, is placed on a heating stage 67. The film carrier 56 is located above the semiconductor chip 59 and is positioned such that leads 55 are located over corresponding bumps 54 (electrode elements 60).
As shown in FIG. 12B, at the second step, the leads 55 are pressed against the bumps 54 by a hot pressure tool 61 so that the leads 55 are bonded to the bumps 54, resulting in an electrical connection between leads 55 and electrode element 60 via bumps 54.
As shown in FIG. 12C, at the third step, the hot pressure tool 60 is removed from the leads 55 bonded to the bumps 54. Thus, a number pair of leads 55 and bumps 54 are bonded at one time in a manner of so called "gang bonding".
In FIGS. 13A and 13B, another method for forming bumps, according to the conventional ball-bonding technology, is shown. First, a golden ball 71 is formed by applying an electric spark to the end of golden wire 70 fed by a capillary tool 72. Second, the thus formed golden ball 71 is pressure bonded to he electrode element 60. Then, the golden wire 70 is cut to leave the golden ball 71 as bump 73 bonded to the electrode element 60, as shown in FIG. 13A. By repeating the above described operations, bumps 60 made by golden ball 73 are formed on every electrode element 73, as shown in FIG. 13B.
In FIGS. 14A and 14B, still another method for forming bumps is shown. In this method, the end portion of a lead 55 is etched to have an end projection 75 formed in a mesa-like shape. This end projection 75 serves as a bump for connection between the lead 55 and the electrode element 60 (not shown) of the semiconductor chip 59 (not shown). However, the end projection 75 made of copper has a hardness greater than the usual bumps made of gold. Therefore, the end projection 75 could damage the oxide layer, under the electrode element 60, of the semiconductor chip 59 when the end projection 75 (leads 55) is pressed against the electrode element 30, resulting in so much degradation and less reliable quality of the semiconductor chip.
With demands for high performance and multi functions of semiconductor chips, the high integration of a plurality of components mounted on a single chip is promoted, causing the semiconductor chip to have multi-leads. Especially such semiconductor chips as micro processors and gate arrays are usually designed according to special specification to satisfy the user's own requirements and are developed in a short lead time. In FIG. 15, a single-point bonding method is shown, which is employed for TAB packaging of such semiconductor chips. In this single bonding method, a bonding tool 61 having a contacting end whose area is almost the same as the electrode is used to press leads 55 of the film carrier 56 against corresponding bumps 54 for the pressure bonding therebetween.
In FIGS. 16A to 16C, the steps for bonding a lead with an electrode of a semiconductor chip, according to the single point bonding method, are shown.
As shown in FIG. 16A, at the first step, the semiconductor chip 59 and film carrier 56 are placed on the heating stage 67 in a manner similar to those described with reference to FIG. 12A.
As shown in FIG. 16B, at the second step, each of the leads 55 is pressed against the bump 54 by a bonding tool 61. At the same time an ultra sonic is applied to the bump 54 so that the bump 54 deforms and bonds with the lead 55, causing the bump to electrically connect with the lead 55. As shown in FIG. 16C, at the third step, the second step is repeated to each corresponding pair of leads 55 and bumps 54 until all pairs are bonded. The single point bonding method can be used regardless of the type of bumps formed on the semiconductor chip. Thus, the single point bonding method can be applied for both technologies of the bump-on-wafer and ball-bonding.
Other than the bonding methods described above, a wire bonding technology is usually used for packaging the semiconductor chip. In the wire bonding technology, however, the electrode element of semiconductor chip and electrode of the package are bonded by a fine metallic wire extending therebetween. The fine metallic wire is made of conductive metals such as gold, aluminum, and copper and has a diameter on the order of several tens of micrometer. Therefore, the process, material, and facilities used by the wire bonding technology are different form those of the above described methods for bonding a semiconductor chip with the film carrier.
However, the bonding technologies described above have the following problems with respect to the TAB packaging of the semiconductor chip. Both in the bump-on-wafer technology and the transferred bump technology (shown in FIGS. 14A and 14B), bumps should be formed on the electrode elements of the semiconductor chip and on leads of the film carrier, respectively, before bonding, further causing the following difficulties.
In the bump-on-wafer technology, the semiconductor chip should be formed with bumps on the electrode elements thereof by vapor depositing and photolithographing methods before the semiconductor chips are cut from the wafer. To achieve this, a photo mask is used for electroplating process of the bump forming. The photo mask has a mask pattern corresponding to the positions of leads and electrode elements. Therefore, it is necessary to prepare photo masks having different mask patterns according to those of semiconductors, resulting in the increase of bump manufacturing cost and the reduction of manufacturability. Thus, it is very difficult to apply TAB packaging to semiconductor chips which with various kind but in a small production lot. Furthermore, since the bump forming process has too much manufacturing steps and complicated manufacturing steps, expensive facilities are required for performing such steps, causing the bump forming cost to be increased. Additionally, the processes of vapor deposition, photolithograph, and etching used for forming bumps causes the semiconductor chip to degrade, resulting in the reduction of yield.
In the transferred bump transfer technology, bumps on the leads are formed by transferring the bumps from the bump forming matrix, requiring the transferring process and facilities. Since the bump forming matrix has bumps formed therein at positions corresponding to the electrodes of the semiconductor chip, it is necessary to prepare the bump forming matrix according to kinds of semiconductor chips, resulting in the increase of bump forming matrix manufacturing cost and the reduction of manufacturability. Thus, it is very difficult to apply TAB packaging to semiconductor chips which with various kind but in a small production lot.
In the ball bump technology, the electrode elements of the semiconductor chip suffer from such stresses as an impact load and two thermal stresses, when the bump is formed thereon and when the lead is bonded with the bump. Such stresses cause the oxide layer under the electrode element to crack, resulting in the reduction of yield.
In case that the end portion of a lead is etched to have an end projection having a mesa-like shape, it is necessary to prepare the special tool and facilities for etching the lead end, resulting in the increase of cost of the film carrier. Additionally, since the lead is made of such a metal approximately three times as hard as gold, the oxide layer under the electrodes are broken and cracked when the end projection is pressed against the electrode for bonding therewith. The lead is plastically deformed and bonded with the electrode. However, the lead is too hard to deform to bond with the electrode with sufficient bonding force, resulting in the reduction of yield.